Pteridophyte species richness in the central Himalaya is limited by cold climate extremes at high elevations and rainfall seasonality at low elevations

Abstract

There is a consensus that climate factors strongly influence species richness along elevation gradients, but which factors are crucial and how they operate are still elusive. Here, we assess the relative importance of temperature‐related versus precipitation‐related variables and the relative importance of extreme climate versus climate seasonality in driving pteridophyte species richness. We used correlation and regression analyses to relate species richness of pteridophytes, and their two major groups (lycophytes, ferns), in fifty 100‐m vertical bands to climatic factors representing different aspects of climatic conditions (general climate, stressful climate, and climate seasonality). Variation partitioning analysis was used to determine the relative importance of each group of climatic factors on species richness. Across the entire elevational gradient, species richness had a parabolic response to mean annual temperature (adjusted R 2 = .87−.91), and a linear response to annual precipitation (adjusted R 2 = .82). Mean annual temperature and annual precipitation in the second‐order polynomial model together explained 96.3%−98.7% of the variation in species richness. The variation in species richness uniquely explained by minimum temperature of the coldest month was much greater than that uniquely explained by temperature seasonality, but the variation in species richness uniquely explained by precipitation during the driest month was much smaller than that uniquely explained by precipitation seasonality. Overall, extreme climate variables explained slightly more variation than did climate seasonality. Our study suggests that pteridophyte richness along the elevational gradient is largely driven by a combination of both temperature‐ and precipitation‐related parameters, although precipitation‐related variables play a slightly stronger role, and that extreme low temperature events (at high elevations) and seasonal precipitation variability (at low elevations) are the strongest determinants of pteridophyte species richness.

Global patterns and climatic determinants of phylogenetic structure of regional fern floras

“…Nevertheless, climate extremes uniquely explained more variation than did climate seasonality for the remaining variation of the phylogenetic metric explained by the two types of climatic variables. This is consistent with a previous study at regional scale in the Himalayas (Qian et al, 2022a), and confirms the importance of extreme climatic events in determining the distributions of plant species in general and of ferns in particular.…”

On the relationship between species diversity and range size

“…temperature variability in our study). Mean annual temperature and temperature variability (seasonality) that were examined in this study are among the six climatic variables that have been considered the most important climatic variables influencing plant distributions (Qian et al, 2022). We did not consider the other four climatic variables (i.e.…”

Spatial patterns and climatic drivers of phylogenetic structure for ferns along the longest elevational gradient in the world

“…Thus, our finding for ferns across a broad elevational gradient is in agreement with their finding for ferns across broad latitudinal and longitudinal gradients in China. Interestingly, previous studies focusing on species richness find that precipitation‐related variables explained more variation in fern diversity than did temperature‐related variables (Kessler 2001, Bickford and Laffan 2006, Kreft et al 2010, Kessler et al 2011a, b, Qian et al 2012, 2021), including ferns across the elevational gradient of the central Himalaya (Qian et al 2022a). We propose that the reason why precipitation‐related variables are more important in driving species richness of ferns whereas temperature‐related variables are more important in driving phylogenetic diversity is linked to phylogenetic niche conservatism.…”

“…First, the abundant rainfall and cloud cover at these elevations lead to optimal conditions for plant growth in general and fern growth in particular. Globally, fern species richness is closely correlated to precipitation, presumably due to optimal environmental conditions allowing for species accumulation over time (Kreft et al 2010, Kessler et al 2011a, b, Khine et al 2019, Weigand et al 2020, Suissa et al 2021, Qian et al 2022a). Second, current diversification rates among ferns are correlated with environmental heterogeneity, the latter of which is highest at mid‐elevations in mountains (Suissa et al 2021).…”

“…It is one of the best elevational gradients for testing various hypotheses on species assembly along thermal and humidity gradients. Previous studies have investigated the relationships of species richness with climatic factors for pteridophytes (including lycophytes and ferns) (Bhattarai et al 2004, Qian et al 2022a) and the relationships of species richness and phylogenetic structure with climatic factors for angiosperms along the elevational gradient (Grytnes and Vetaas 2002, Vetaas and Grytnes 2002, Bhattarai and Vetaas 2003, Qian et al 2019b, Rana et al 2019), but the relationships between phylogenetic structure and climatic factors for ferns along this elevational gradient have not been studied. For angiosperms, these studies have found a triphasic (zigzag) pattern of phylogenetic relatedness which has been interpreted as resulting from the interaction of geophysical and ecoevolutionary processes, with plate tectonics, niche conservatism and trait convergence being the crucial factors at low, middle and high elevations, respectively.…”